1. Field of the Invention
The present invention relates to a process for treating NF3 gas that is useful as a dry etching gas and cleaning gas in processes for producing LSI, TFT, and solar cell and in an electron photographic process.
2. Description of the Invention
NF3 is a toxic gas having a TLV of 100 ppm that is extremely stable in air and essentially insoluble in water. In the case of using this substance, it is necessary at all times to remove residual NF3 present in exhaust gas. Since NF3 is extremely chemically stable at temperatures near room temperature and is also insoluble in water, it cannot be processed by ordinary gas absorption processes in its original state. Consequently, the following process has been proposed in Japanese Patent No. 1538007 (Japanese Patent Provisional Publication No. 61-204025), in which NF3 is reacted with a substance that converts NF3 into a fluoride gas that easily reacts with water and alkaline solution, followed by treating the resulting fluoride gas with a normal gas absorption process. The Japanese Patent discloses a process wherein NF3 is reacted with Si, B, W, Mo, V, Se, Te, Ge and their non-oxidizing solid compounds that are used as the converting substance.
Although the above NF3 treatment process is effective for converting NF3 into an easily treated gas compound, a characteristic reactor taking the NF3 into consideration was not proposed with respect to the reactor for reacting and treating a large amount of NF3. Namely, the above Patent only proposes a single flow type of fixed bed reactor as equipment for contacting gaseous NF3 with a solid compound.
The fixed bed reactor described in the present specification refers to a reactor having a cylindrical outer tube in which a fixed bed, filled with a solid compound such as a metal element that reacts with NF3 throughout an ordinary cylindrical reactor, is disposed. The fixed bed is heated as necessary followed by introducing gas from one end of the cylinder, contacting and reacting the gas with a metal element and so forth inside the cylindrical tube, and discharging the gas from the other end of the tube. This form of the reactor is that which has been known since long ago. In addition, various types of NF3 detoxification technologies taking into consideration new reaction systems other than the above reaction system have been disclosed as being disclosed in Japanese Patent Publication No. 2-30731 and Japanese Patent provisional Publication No. 7-155540. In such technologies, the fixed bed reactor of the gas flow type is still used. Thus, the reactors that provide an effective setting for an NF3 detoxification reaction have not yet discovered.
Now, treatment of NF3 gas is accompanied by the generation of extremely large amounts of heat from the reaction. Namely, its standard formation enthalpy is xe2x88x92127 kJ/mol (xe2x88x9242 kJ per fluorine atom), and in the case of SiF4 gas being obtained as the product of the action of metal Si, for example, since the standard formation enthalpy of SiF4 is xe2x88x921615 kJ/mol (xe2x88x92404 kJ per fluorine atom). The difference between the two enthalpies are the amount of heat generated accompanying reaction (362 kJ per fluorine atom), which demonstrates that NF3 detoxification reaction is accompanied by the generation of an extremely large amount of heat. Even though there may be some difference in the amount of heat generated in the case that the other substance than Si such as B or W, or if C is selected as a reacting metal element: however, it is intrinsically a reaction that is accompanied by the generation of a large amount of heat.
In the case of conducting a gas-solid reaction using a reactor or reaction tube of the type in which gas is allowed to flow over a fixed bed, the reaction initially occurs in the zone on the inlet side of the initial reaction tube, and as chemical is consumed, the reaction zone gradually moves to the outlet side. Since the flow of gas inside the reactor is so-called piston flow, there are many cases in which the reaction always occurs in a special location inside the reaction tube in this manner, while other portions of the reaction tube merely fulfill the role of a gas pathway and are not involved in the reaction itself. Moreover, due to the low rate of heat transfer of the fixed bed, it cannot be said to be suited for efficiently discharging the reaction heat generated locally inside the reactor in this manner outside the system.
For these reasons, when an NF3 detoxification reaction is carried out with a fixed bed gas flow system for a reaction that generates a large amount of heat, the local temperature that results from the reaction ends up becoming extremely high. Consequently, the amount of NF3 that be treated per unit time cannot be increased relative to the volume of the reactor.
Moreover, there has been proposed a process in which the concentration of supplied NF3 is diluted with an inert gas (such as N2) for the purpose of lowering the temperature of the formed gas. However, this process increases the volumetric flow of all gas resulting in a shortening of retention time, and therefore is not effective as a means of improving the NF3 treatment rate per reactor volume. Moreover, even if a large reactor is attempted to be designed having a larger NF3 treatment rate, there is a limit on the size of the reaction tube diameter for ensuring heat transfer in the radial direction. Ultimately, in order to provide NF3 treatment volume, a plurality of small diameter reactors must be arranged in parallel, and in any case, fixed bed gas flow systems had the problem of being disadvantageous in terms of equipment cost.
In addition, in the case of using Si, for example, in the reaction between NF3 and Si, a relatively large amount of heat is generated on the order of 1,086 kJ/mol. Consequently, this invites a local temperature rise and overheating in conventional tubular apparatuses of the fixed bed type, thereby placing a limit on the amount (concentration) of NF3 supplied, and the limit of that supplied concentration is 5 vol %. In addition, the actual limit on the tube diameter of a fixed bed system is 150 A (according to Japanese Industrial Standard) corresponding to an outer diameter of 165.2 mm. Namely, it was necessary to accompany treatment of NF3 at 5 NL/min with a diluting gas (N2) at 100 NL/min. For this reason, fixed bed systems are not suited for treatment of highly concentrated NF3 or large amounts of NF3.
In addition, in the case of fixed bed systems, treatment capacity has been observed to decrease when air or oxygen is present. Consequently, there is a need for an NF3 treatment process that allows treatment of highly concentrated NF3, does not result in a decrease in treatment rate even in the presence of, for example oxygen (air) in the NF3, and is able to ensure a certain degree of treatment volume per unit time.
As a result of conducting earnest studies in consideration of the above-mentioned problems, the inventors of the present invention have found that highly concentrated and large amounts of NF3 gas can be treated by creating a setting for gas flow that prevents local overheating of the fixed portion of a reactor, rapidly transports generated heat to the wall of the reactor with the flow of gas, and provides as rapid a gas flow as possible along the reactor wall to promote transfer of heat between the gas phase and solid wall in the vicinity of the reactor wall, thereby leading to completion of the present invention.
An aspect of the present invention resides in a process for treating NF3, comprising the following step: (a) preparing a first reactor including agitator blades for agitating gas in the first reactor and generating a flow of the gas, and a gas flow guide tube for efficiently circulating and dispersing the gas flow generated by the agitator blades in a space of the first reactor; (b) stationarily placing at least one substance selected from the group consisting of a metal and a metal compound within a first reactor, the metal being at least one metal selected from the group consisting of Si, B, W, Mo, V, Se, Te and Ge, the metal compound being at least one metal compound selected from the group consisting of solid compounds of Si, B, W, Mo, V, Se, Te and Ge; (c) introducing a gas containing NF3 into the first reactor to react the introduced gas with at least one substance of the metal and the metal compound at a temperature ranging from 400 to 900xc2x0 C. upon operating the agitator blades of the first reactor so as to form a fluoride gas; and (d) capturing the fluoride gas.
The above process may further comprises the steps of (e) connecting a second reactor in series with and at a side downstream of the first reactor, the second reactor having a fixed bed including at least one substance of a metal and a metal compound within a first reactor, the metal being at least one metal selected from the group consisting of Si, B, W, Mo, V, Se, Te and Ge, the metal compound being at least one metal compound selected from the group consisting of solid compounds of Si, B, W, Mo, V, Se, Te and Ge; and (f) introducing gas discharged from the first reactor to the second reactor so as to react the gas with the at least one substance of the metal and the metal compound within a temperature ranging from 400 to 900xc2x0 C.
Another aspect of the present invention resides in a system for treating NF3. The system comprises a reactor which includes an outer tube into which a gas containing NF3 is supplied. Agitator blades are disposed inside the outer tube for agitating the gas and generating a flow of the gas. A gas flow guide tube is disposed inside the outer tube to efficiently circulate and disperse the gas flow generated by the agitator blades in a space of the outer tube. Additionally, at least one substance selected from the group consisting of a metal and a metal compound, disposed inside the gas flow guide tube. The metal is at least one metal selected from the group consisting of Si, B, W, Mo, V, Se, Te and Ge. The metal compound is at least one metal compound selected from the group consisting of solid compounds of Si, B, W, Mo, V, Se, Te and Ge. Additionally, a heater is disposed outside the outer tube to heat the space inside the outer tube at a temperature ranging from 400 to 900xc2x0 C.
According to the NF3treatment process and system of the present invention, gas containing NF3 in a large amount and/or at a high concentration can be adequately removed while performing the treatment process safely without the formation of explosive gas by-products.